This invention is in the field of semiconductor processing, and more particularly in the field of photolithography.
In semiconductor processing, photolithography is the process of defining particular features on the surface of a semiconductor wafer. The feature is typically defined with a patterned exposure into a photosensitive material that has been previously deposited on the wafer surface. In a typical photolithographic system, a master pattern image in the form of a photomask or reticle is replicated across the surface of the wafer. The image is typically projected onto the photosensitive material (e.g. photoresist) through a lens system. The quality of the image, and hence the quality and reliability of the resulting feature on the semiconductor surface, is directly related to mechanical parameters such as the spacing and orientation of the lens system relative to the chuck and stage upon which the wafer is placed.
Lithography equipment typically includes a mechanism for fixing the wafer onto a chuck that rests on a movable stage. If the chucking is not done correctly, or if contamination exists on the chuck or the wafer backside, the wafer will not lie flat on the surface of the chuck. This often results in resolution failures as the system has difficulty focusing the image in the area of the topographical feature caused by the chucking error. The local tilt of the wafer surface in the area of the undesired topographical feature can cause misformation of the projected image. The imaging errors can result in the scrapping of one or more die of the wafer, or indeed one or more die of multiple wafers or multiple lots of wafers if a repeatable and persistent error goes undetected.
Photolithographic tools include a stage that is capable of highly accurate and precise positioning so that the wafer surface is in position to receive a focused projected image. In addition to movement up, down, and side-to-side, a stage must also tilt when necessary to image a desired feature. FIG. 1 shows a prior art stepper or step and scan photolithographic tool. The stage 10, wafer chuck 12, and wafer 14 are moved relative to the reticle 16. Other tool components include the pellicle 18, the lens system 20, and the light source 22. A stepper tool typically exposes the entire reticle onto the wafer at once, whereas a step and scan tool exposes the image by scanning a slit over the surface of the reticle. In either case, the stage must be positioned accurately and precisely for proper exposure of the desired feature in the photoresist. FIG. 2 illustrates the movements of a typical stage.
The photolithographic tool usually achieves focus on the wafer by analyzing an alignment key such as a plus-shaped mark or bulls-eye on the wafer. A typical stage is monitored with laser interferometers with resolution on the order of less than one nanometer as the tool seeks to position the wafer for optimum focus and resolution of the projected image. The degree of translation of the stage along x-, y-, and z-axes for up, down, and side-to-side movements as well as degree of stage tilt as the tool seeks to focus the projected image on the wafer surface are routinely measured and recorded on a die-by-die basis as the image is replicated across the wafer.
FIGS. 3a to 3d are depictions of light impinging upon the photoresist layer on a semiconductor wafer. FIG. 3a is an ideal situation in which the light paths striking the photoresist surface all travel equal lengths. FIG. 3b shows the effects of a wafer surface that is inclined relative to the light source. FIG. 3c shows the effects of warpage of the wafer or perhaps dishing of the photoresist layer. FIG. 3d shows the effects of either a particle on the backside of the wafer or a defect such as a scratch in the photoresist. Note that in the cases of FIGS. 3b, 3c, and 3d, the light paths are not equal, a fact which typically results in poor reproduction of the reticle image in the photoresist. To counteract this, a typical photolithographic tool attempts to tilt the stage relative to the light source as it steps or scans over the inclined or warped surface, or over the undesirable topographical feature, as the case may be.